High and low side driver tests for airbag module

ABSTRACT

A method of testing a high-side driver and a low-side driver in an airbag squib circuit includes preliminary testing of squib resistance and squib leakage for a plurality of trials. Next, one of the drivers is turned on while keeping the other one of the drivers off. A current-limited power supply supplies an intermediate voltage to a squib terminal and the voltage at the terminal is continuously compared with a predetermined voltage range which includes the intermediate voltage. The one driver is turned off in response to the voltage at the point being outside the predetermined voltage range, thereby detecting that the one driver is operating properly. If the voltage at the point remains in the predetermined voltage range for a predetermined time period, then the one driver is turned off and an indication is made that the one driver has failed. If the first driver passed, then the other driver is tested in the same manner.

BACKGROUND OF THE INVENTION

[0001] The present invention relates in general to circuits fordeploying airbag igniters or squibs, and, more specifically, to circuitdiagnostics for testing proper operation of drivers that supplyelectrical energy to ignite the squibs.

[0002] The main components of an airbag supplemental restraint systemused in motor vehicles include an inflatable bag, a propellant source(e.g., sodium azide pellets), an igniter or squib to initiate burning ofthe propellant source, at least one crash sensor, and an electroniccontrol module for determining when to deploy an airbag and sending adeployment pulse to the igniter. The airbag, propellant, and igniter arecontained in an airbag module (e.g., within a steering wheel for adriver airbag). The sensor can be packaged separately or can becontained within the electronic control module.

[0003] The control module performs self-diagnostic monitoring of thesupplemental restraint system each time the system is turned on (e.g.,every time a vehicle is started). Any potential performance problems areidentified and a warning light is illuminated so that the driver knowsthat the system needs to be serviced.

[0004] It is known to perform diagnostic monitoring of the electricalconnection of the squib elements, squib resistances, and electricalleakage in the squib circuits, among other tests. When performingelectrical testing involving the squibs, care must be taken to avoidapplication of any current to a squib that could cause inadvertentdeployment of the airbag. Due to the cost of replacing an airbag moduleand the loss of supplemental protection until replacement occurs,diagnostic monitoring should not increase the chances of inadvertentdeployment.

[0005] A very desirable test to be able to perform is a driver test inwhich a squib driver circuit can be activated in a test mode withoutigniting the squib. Such a test can verify that a semiconductor switchin series with the squib element itself will conduct as intended duringan actual deployment event. However, such a test has been problematicsince the activation of the switch partially completes the deploymentcircuit. If certain other faults exist, or if switch activation is notimplemented properly, unintended deployments can occur.

SUMMARY OF THE INVENTION

[0006] The present invention has the advantage that high-side andlow-side drivers in series with a squib element can be tested whileavoiding inadvertent airbag deployment.

[0007] In one aspect, the present invention provides a method of testinga high-side driver and a low-side driver in an airbag squib circuit. Theairbag squib circuit includes a squib element coupled between thehigh-side driver and the low-side driver. The high-side drivercontrollably provides a high-side voltage to one side of the squibelement and the low-side driver controllably provides a low-side voltageto the other side of the squib element. A resistance of the squibelement is tested for a resistance value within a predeterminedresistance range. A current leakage associated with said squib elementis tested to determine whether it is over a leakage threshold. Anintermediate voltage from a weak power supply is supplied to a point inthe airbag squib circuit between the high-side driver and the low-sidedriver. One of the drivers is turned on while keeping the other one ofthe drivers off. A voltage at the point is continuously compared with apredetermined voltage range which includes the intermediate voltage. Theone driver is turned off in response to the voltage at the point beingoutside the predetermined voltage range, thereby detecting that the onedriver is operating properly. If the voltage at the point remains in thepredetermined voltage range for a predetermined time period, then theone driver is turned off and an indication is made that the one driverhas failed.

[0008] Unless there is a failure, the other driver is then turned onwhile keeping the one driver off. A voltage at the point is continuouslycompared with the predetermined voltage range. The other driver isturned off in response to the voltage at the point being outside thepredetermined voltage range, thereby detecting that the other driver isoperating properly. If the voltage at the point remains in thepredetermined voltage range for the predetermined time period, then theother driver is turned off and an indication is made that the otherdriver has failed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a block diagram showing an electronic controller andintegrated circuit for an airbag supplemental restraint system.

[0010]FIG. 2 is a schematic diagram showing apparatus for performing thehigh-side and low-side driver tests of the present invention.

[0011]FIG. 3 is a flowchart showing a preferred method of determiningthat certain conditions are not present that would prevent a high-sideand a low-side driver test.

[0012]FIG. 4 is a flowchart of a preferred embodiment of the driver testof the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0013] Referring to FIG. 1, a control module 10 includes amicrocontroller 11 coupled to an airbag firing application-specificintegrated circuit (ASIC) 12. ASIC 12 is connected to a pair of remotesquib elements 13 and 14 through appropriate vehicle wiring.Microcontroller 11 is connected to a crash sensor (not shown) such as anaccelerometer to determine when a particular crash condition isoccurring in which an airbag should deploy. ASIC 12 can be comprised ofa TLE6712 Dual Firing Airbag IC available from Infineon Technologies AGof Munich, Germany, for example.

[0014] Each squib element is connected in a respective deployment loopcomprising a driver circuit for providing an igniting pulse to itsrespective squib in response to an appropriate command frommicrocontroller 11. A two loop ASIC is shown in FIG. 1, although thepresent invention can be used advantageously with any number of loops. Afirst loop for squib element 13 includes a supply of a deploymentvoltage V_(deploy), a first driver 15, and a connection to ground GND. Asecond loop for squib element 14 includes a connection to V_(deploy), asecond driver 16, and a connection to ground GND. V_(deploy) may be adirect connection to a high capacity supply, such as a vehicle storagebattery V_(batt) or a source of stepped up battery voltage, to ensuresufficient energy for a deployment event. ASIC 12 also receives aregulated voltage V_(cc) for supplying power (e.g., 5 volts) to on-chipcomponents.

[0015] ASIC 12 further includes a serial peripheral interface (SPI)block 17 for receiving and decoding commands from microcontroller 11.The commands include a deployment command for each loop, and measurementand self-diagnostic test commands, for example. Deployment control block18 includes logic to control the proper sequence of events to ignite asquib element via the driver(s) 15 and 16. Measurement block 19 performsvarious diagnostic measurements, such as squib resistance measurementsas known in the art.

[0016] Each driver within a deployment loop comprises a high-side and alow-side semiconductor switch as shown in FIG. 2 such that both switchesmust be turned on in order to complete an igniting circuit through thesquib element. FIG. 2 shows only one deployment loop, although theinvention is applicable to any number of deployment loops. In this onedeployment loop, a MOSFET 25 has one output connected to a power supplyterminal V_(deploy) and its other output connected to a first squibterminal 26. A MOSFET 27 has one output connected to ground and itsother output connected to a second squib terminal 28. Squib terminals 26and 28 are connected to squib element 13 by a vehicle wiring harness andconnectors.

[0017] The gate terminal (i.e., input) of MOSFET 25 is connected to theoutput of an AND-function 30 and the gate terminal of MOSFET 27 isconnected to the output of an AND-function 31. Each AND-function has oneinput connected to receive an Enable signal, which may be provided as aseparate control signal from the microcontroller, for example. A secondinput of AND-function 30 receives a

high-side switch on

signal (HSx ON signal), which may be provided in response to acorresponding SPI command from the microcontroller, for example. Asecond input of AND-function 31 receives a

low-side switch on

signal (LSx ON signal). The

x

in HSx ON and LSx ON is an index to refer to each deployment loop in theASIC. The Enable signal and the HSx ON and LSx ON signals must have ahigh logic level in order to turn on each respective MOSFET.

[0018] Not shown in FIG. 2 is the conventional circuitry withinmeasurement block 19 of FIG. 1 which performs squib resistancemeasurements. This circuitry is also connected to squib terminals 26 and28 and it typically is comprised of a self-calibrating circuit thatmeasures a voltage drop across a squib element. A normal resistance fora squib element is typically about 2 ohms. A resistance measurementgreater than about 4 ohms or less than about 1 ohm usually indicates afault in the squib element. Ignition of a typical squib element requiresabout 1 amp of current in an ignition pulse. Resistance testing istypically performed using about 50 milliamps passing through the squibelement.

[0019]FIG. 2 shows circuit elements used in the present invention fortesting both 1) squib leakage, and 2) highside and low-side driverperformance. A voltage regulator 32 nominally provides an intermediatevoltage to squib terminal 26 (although it could alternatively beconnected to squib terminal 28). In a preferred embodiment, theintermediate voltage is ½ V_(batt). Regulator 32 has a very limited(i.e., weak) current capacity so that it can only keep the voltage onsquib terminal 26 at ½ V_(batt) if nothing else in the circuit ispulling squib terminal 26 to ground, battery voltage, or some othervoltage. The maximum current from regulator 32 is insufficient to ignitethe squib element.

[0020] The voltage present at squib terminal 26 (or alternativelyterminal 28) is compared to a predetermined voltage range (whichincludes the nominal voltage from regulator 32) by a detector 33comprising a comparator 34, a comparator 35, and an OR-function 36.Terminal 26 is connected to the inverting input of comparator 34 and tothe noninverting input of comparator 35. The noninverting input ofcomparator 34 is connected to a first reference voltage, equal to about¼ V_(batt) in this preferred embodiment. The inverting input ofcomparator 35 is connected to a second reference voltage, equal to about¾ V_(batt) in this preferred embodiment. The outputs of comparators 34and 35 are connected to respective inputs of OR-function 36. The outputof OR-function 36 provides a Leak signal. Detector 33 establishes apredetermined voltage range from ¼ V_(batt) to ⅓ V_(batt). During asquib leakage test, regulator 32 is turned on and if the voltageappearing at squib terminal 26 stays at about ½ V_(batt) then theoutputs of comparators 34 and 35 stay at a low logic level. Because ofthe low logic level input signals to OR-function 36, the Leak signalstays at a low logic level, thereby indicating that there is no squibleakage (e.g., no shorts to ground or to battery). If the voltage iseither pulled high (i.e., over ¾ V_(batt)) or low (i.e., under ¼V_(batt)) outside the predetermined range, then one of the comparatoroutputs switches to a high logic level and the Leak signal also goeshigh, thereby indicating a squib leakage fault. The Leak signal is sentto the microcontroller which keeps track of specific fault occurrencesand generates a fault indication, such as turning on amalfunction-indicator light.

[0021] The present invention makes use of voltage regulator 32 and theLeak signal to perform high-side driver and low-side driver testing,provided that the squib resistance and squib leakage tests are passed.If these tests are not passed then 1) the risk of an inadvertentdeployment being caused when just one of the switches is closed would beincreased, and 2) a driver test would have little incremental valuesince the deployment loop will already be faulted.

[0022] Assuming the resistance and leakage tests are passed, thenvoltage regulator 32 is turned on (if not already on) and one driver isturned on to test it. If the switch logic and MOSFET perform asintended, then squib terminal 26 will have a path either to V_(deploy)or to ground which overcomes the ability of regulator 32 to keep theterminal at ½ V_(batt). Consequently, the Leak signal will go to a highlogic level to indicate proper functioning of the respective driver.

[0023] To most effectively limit the possibility of inadvertent airbagdeployment during a driver test, the present invention utilizes theoverall method shown in FIGS. 3 and 4.

[0024] Preliminary to the actual driver tests, the present inventiontests all deployment loops in use on an ASIC for proper squib resistanceand absence of squib leakage. For high reliability, these tests areconducted a plurality of times (e.g., 10 times) for each deployment loopin a round-robin fashion. If at any time, a resistance test or a leakagetest indicates a fault, then the 10 testing rounds are re-started. Thisre-starting continues indefinitely until 10 consecutive rounds ofresistance and leakage tests are passed for all deployment loops.

[0025] In step 40, an index

Loop

for keeping track of a deployment loop being tested is initialized toone and an index

Trial

for keeping track of successful testing rounds is initialized to zero.In step 41, a squib leakage test is performed for the current deploymentloop identified by index Loop (e.g., loop 1 for the first execution ofstep 41). If leakage is detected (i.e., the test is failed) then areturn is made to step 40 to begin a new attempt to make it through 10rounds of tests without a failure. If the leakage test is passed, thenthe resistance of the squib element in the current deployment loopidentified by index Loop is measured in step 42 and compared to itspermissible values. If the squib resistance fails the test, the methodreturns to step 40. If the resistance test is passed, then a check ismade in step 43 to determine whether Loop equals the number of loopsbeing used in the ASIC (i.e., whether the last loop has been tested). Ifnot, then Loop is incremented by 1 in step 44 and a return is made tostep 41. If the current loop was the last loop, then the index Trials isincremented by one in step 45. A check is made in step 46 to determineif Trials equals 10 (i.e., if 10 consecutive successful round-robin testtrials have been completed). If not, then Loop is reset to 1 in step 47and a return is made to step 41. Otherwise, the method progresses to thedriver test phase via a point A.

[0026] Specific driver testing of a preferred embodiment is shown inFIG. 4 for one deployment loop. The low-side driver is tested first. Instep 50, the LSx ON signal goes high in response to an SPI command fromthe microcontroller while the Enable signal remains or is set to OFF(i.e., logic low). If the AND-function logic element is functioningproperly then there should be no change in the state of the low-sideswitch. A check is made in step 51 to determine if the Leak signal goeshigh during a predetermined delay period (preferably equal to about 1millisecond, for example). If it did, then a low-side driver fault isindicated in step 52 and the LSx ON signal is reset to a low level.

[0027] If the Leak signal stayed low in step 51, then the Enable signalis turned on by the microcontroller in step 53, a time counter isstarted, and the microcontroller begins to continuously monitor the Leaksignal for a transition to a high logic level. An important goal of thepresent invention is to minimize the amount of time that a driver switchis turned on. Therefore, the microcontroller repeatedly and rapidlyinspects the Leak signal. If a particular inspection determines that theLeak signal has not gone high, then a check is made in step 55 todetermine whether the time counter has reached a predetermined timeperiod (preferably equal to about 115 microseconds, for example). Aproperly operating driver would normally trigger the leak detectorcircuit in less than about 50 microseconds, but to allow for processvariations, capacitance on a squib line, or other factors, a time periodof 115 microseconds is allowed. If 115 microseconds have not yetexpired, then a return is made to step 54. If 115 microseconds haveelapsed, then a low-side driver fault is indicated in step 56 and theLSx ON signal and the Enable signal are reset to OFF.

[0028] As soon as a Leak signal having gone to a high logic level isdetected in step 54, the microcontroller immediately takes the Enablesignal to its unasserted (i.e., OFF) level in step 57. This turns offthe low-side driver and removes ground from the squib terminal. At thispoint, the low-side driver has passed the driver test and the LSx ONsignal is changed to low via an SPI command.

[0029] Also in step 57, the HSx ON signal is changed to high via an SPIcommand in order to initiate testing of the high-side driver. The Leaksignal is inspected by the microcontroller in step 58 to verify noturning on of the high-side switch without the Enable signal beingasserted. If the Leak signal is detected then a high-side driver faultis indicated in step 60 and the HSx ON signal is reset low.

[0030] If no Leak signal is detected in step 60, then the high-sidedriver is turned on and tested in steps 61-64 in the same manner as thelow-side driver. If proper driver operation is detected in step 62, thenthe Enable signal and the HSx ON signal are reset to OFF and the weakvoltage regulator is turned off in step 65. Any remaining high-side andlow-side drivers to be tested in other loops are then tested in step 66in the same manner. Thereafter, normal operation of the ASIC proceeds.

[0031] The present invention achieves a variable turn-on time for eachdriver. In other words, a driver is on only for as long as necessary toverify its proper operation. The transition of the Leak signal initiatesthe turning off of the corresponding driver. Therefore, any applicationof current to a corresponding squib is kept as short as possible.Furthermore, the requirement for a plurality of consecutive successfulresistance and leakage tests increases the likelihood of being able toconduct a driver test without an inadvertent airbag deployment.

What is claimed is:
 1. A method of testing a high-side driver and alow-side driver in an airbag squib circuit, said airbag squib circuitincluding a squib element coupled between said high-side driver and saidlow-side driver, said high-side driver controllably providing ahigh-side voltage to one side of said squib element and said low-sidedriver controllably providing a low-side voltage to the other side ofsaid squib element, said method comprising the steps of: testing aresistance of said squib element for a resistance value within apredetermined resistance range; testing for a current leakage associatedwith said squib element; if said resistance and current leakage testsare passed, then supplying an intermediate voltage from a weak powersupply to a point in said airbag squib circuit between said high-sidedriver and said low-side driver; turning on one of said drivers whilekeeping the other one of said drivers off; continuously comparing avoltage at said point with a predetermined voltage range including saidintermediate voltage; turning off said one driver in response to saidvoltage at said point being outside said predetermined voltage range,thereby detecting that said one driver is operating properly; if saidvoltage at said point remains in said predetermined voltage range for apredetermined time period, then turning off said one driver andindicating that said one driver has failed; turning on said other driverwhile keeping said one driver off; continuously comparing a voltage atsaid point with said predetermined voltage range; turning off said otherdriver in response to said voltage at said point being outside saidpredetermined voltage range, thereby detecting that said other driver isoperating properly; if said voltage at said point remains in saidpredetermined voltage range for said predetermined time period, thenturning off said other driver and indicating that said other driver hasfailed.
 2. The method of claim 1 further comprising the step of:repeating said current leakage testing for a plurality of trials until apredetermined number of consecutive trials show an absence of currentleakage over said leakage threshold, wherein said high-side driver andsaid low-side driver are only tested if said predetermined number isobtained.
 3. The method of claim 1 further comprising the step of:repeating said resistance testing for a plurality of trials until apredetermined number of consecutive trials show a resistance valuewithin said predetermined resistance range, wherein said high-sidedriver and said low-side driver are only tested if said predeterminednumber is obtained.
 4. The method of claim 1 wherein said intermediatevoltage is substantially equal to one-half of said high-side voltage,wherein said predetermined range is from about one-fourth of saidhigh-side voltage to about three-fourths of said high-side voltage, andwherein said low-side voltage is substantially equal to ground.
 5. Themethod of claim 1 wherein said predetermined time period issubstantially equal to about 115 microseconds.
 6. Apparatus for firingan airbag squib comprising: a high-side voltage supply terminal forcoupling to a high-side voltage; a high-side semiconductor switch havingan input terminal and a pair of output terminals, one of said outputterminals being coupled to said high-side voltage supply terminal andthe other of said output terminals being coupled to a first squibterminal; a ground terminal for coupling to ground; a low-sidesemiconductor switch having an input terminal and a pair of outputterminals, one of said output terminals being coupled to said groundterminal and the other of said output terminals being coupled to asecond squib terminal; a first logic element having first and secondinputs and an output, said output coupled to said input terminal of saidhigh-side semiconductor switch, said first input receiving an enablesignal and said second input receiving a high-side activate signal; asecond logic element having first and second inputs and an output, saidoutput coupled to said input terminal of said low-side semiconductorswitch, said first input receiving said enable signal and said secondinput receiving a low-side activate signal; a voltage regulator coupledto one of said first or second squib terminals, said voltage regulatorproviding a current-limited power supply having a nominal voltage whichis intermediate of said high-side voltage and said ground; a voltagedetector coupled to a selected one of said first or second squibterminals for detecting whether a resultant voltage on said selectedsquib terminal is within a predetermined voltage range including saidnominal voltage; and a controller for 1) activating said voltageregulator, 2) generating said enable signal and said high-side activatesignal to turn on said high-side semiconductor switch while keeping saidlow-side semiconductor switch turned off, 3) continuously monitoringsaid resultant voltage using said voltage detector, 4) ceasing saidenable signal or said high-side activate signal to turn off saidhigh-side semiconductor switch in response to said resultant voltagebeing outside said predetermined voltage range, thereby detecting thatsaid high-side semiconductor switch and said first logic element areoperating properly, and 5) if said resultant voltage remains in saidpredetermined voltage range for a predetermined time period, thenceasing said enable signal or said high-side activate signal to turn-offsaid high-side semiconductor switch and indicating a failure.
 7. Theapparatus of claim 6 wherein said controller is further adapted for 1)generating said enable signal and said low-side activate signal to turnon said low-side semiconductor switch while keeping said high-sidesemiconductor switch turned off, 2) continuously monitoring saidresultant voltage using said voltage detector, 3) ceasing said enablesignal or said low-side activate signal to turn off said low-sidesemiconductor switch in response to said resultant voltage being outsidesaid predetermined voltage range, thereby detecting that said low-sidesemiconductor switch and said second logic element are operatingproperly, and 4) if said resultant voltage remains in said predeterminedvoltage range for a predetermined time period, then ceasing said enablesignal or said low-side activate signal to turn-off said low-sidesemiconductor switch and indicating a failure.
 8. The apparatus of claim6 wherein said controller is further adapted to perform a leakage testwherein said controller 1) activates said voltage regulator, 2) checksthat said resultant voltage remains within said predetermined voltagerange, and 3) repeats item 2 until said resultant voltage stays withinsaid predetermined voltage range for a consecutive, predetermined numberof trials.
 9. The apparatus of claim 6 wherein said controller isfurther adapted to perform a squib resistance test wherein saidcontroller 1) checks that a resistance of said squib element remainswithin a predetermined resistance range, and 2) repeating item 1 untilsaid resistance is within said predetermined voltage range for aconsecutive, predetermined number of trials.
 10. The apparatus of claim6 wherein said first and second logic elements are comprised ofAND-functions.
 11. The apparatus of claim 6 wherein said nominal voltageis substantially equal to one-half of said high-side voltage.
 12. Theapparatus of claim 6 wherein said predetermined time period issubstantially equal to about 115 microseconds.